Thermotropic liquid crystalline polyesters (TLCPs) are high-performance materials with unique and remarkable combination of properties, such as excellent melt forming characteristics, high heat deflection temperature (HDT) and high mechanical strength, etc. They are useful as molding resins for general purpose uses, and in particular, for applications in the electrical and electronics industries. Also they can be useful as fiber and film for various applications, and as modifier for improving the properties of conventional polymers.
Polyester TLCPs derived from p-hydroxybenzoic acid (HBA), 6-hydroxy-2-naphthoic acid (HNA), terephthalic acid (TA), and hydroquinone (HQ) are known, see for instance U.S. Pat. No. 4,219,461. However, the relationship between composition and melting temperature of the TLCPs is not mentioned in the patent, and the melting temperature of the TLCP actually made is not high enough and then high HDT cannot be expected.
U.S. Pat. No. 4,318,841 describes a TLCP with repeat units derived from HBA, HNA, TA and resorcinol (m-HQ). U.S. Pat. No. 4,370,466 describes a TLCP with repeat units derived from HBA, HNA, IA (isophthalic acid), and HQ. In both cases, the repeat units with non-symmetrical in structure are introduced into main chain. In U.S. Pat. No. 4,473,682, 4,4′-biphenol (BP) was used to synthesize TLCPs in combination of HBA, HNA and TA, and the resulting TLCPs may have better thermal properties. U.S. Pat. No. 5,015,722 reported the synthesis of the TLCPs derived from HBA, HNA, TA, BP and HQ, which have a high HDT and a relatively low melting temperature. All these patents do not mention the correlation between composition and melting temperature of the polyesters.
U.S. Pat. No. 6,306,946 reported the preparation of TLCPs derived from HBA, TA, HQ and 2,6-naphthalenedicarboxylic acid (NDA). An empirical equation was obtained on the basis of composition and melting point of the polymers, and a contour map of melting point vs. polymer composition was prepared. But TLCP mentioned in the patent has a significant difference in structure from the TLCP of this invention.
For TLCP applications in the electrical and electronics industries, the melting temperature of TLCP is usually required to be high enough to insure high HDT and low enough for better processibility. A correlation between composition and melting temperature is of interest to establish the optimum compositional range for better processibility and high HDT. On the other hand, when TLCP is used to modify conventional polymers, the melting point has to be much lower to match that of the polymers, so that melt mixing can be performed. In the present paper, therefore, a series of TLCPs were synthesized with units derived from p-hydroxybenzoic acid (HBA), 6-hydroxy-2-naphthoic acid (HNA), terephthalic acid (TA) and hydroquinone (HQ) in a wide range of compositions, and the correlation between melting temperature and composition of the polyesters is established. According to the correlation, it is easier to design composition and to synthesize TLCP with desired melting temperature.